Camera lens of a large aperture ratio having long back focus



JbU-Qbl SR -Lmun nux x ea osT April 21, 1970 YOSHIYUKI SHIMIZU 3,507,558

CAMERA LENS OF A LARGE APERTURE RATIO HAVING LONG BACK FOCUS Filed Oct.20. 1966 United States Patent 3,507,558 CAMERA LENS OF A LARGE APERTURERATIO HAVING LONG BACK FOCUS Yoshiyuki Shimizu, Tokyo, Japan, assignorto Nippon Kogaku K.K., Tokyo, Japan, represented by Hiroshi Shirahama, acorporation of Japan Filed Oct. 20, 1966, Ser. No. 588,212 Claimspriority, application Japan, Oct. 22, 1965,

til/64,423 Int. Cl. G02b 9/08, 9/62, 11/32 US. Cl. 350-210 2 ClaimsABSTRACT OF THE DISCLOSURE This invention relates to a camera lens of alarge aperture ratio having a long back focus.

The wide angle lenses used for a single lens reflex camera containing apivotable mirror are generally of the so called back telescopic type.These lenses have a small aperture ratio resulting in a darkened view inthe finder and a large depth of field. Focusing of the object is donewith some difliculty, making a lens of a larger aperture ratio moredesirable.

It is an object of the present invention to provide a lens having alarge a'perture ratio.

In accordance with the present invention, a wide angle lens of largeaperture ratio is provided having a long back focus, the aperture ratiothereof being over 1:2, the angle of field thereof being over 60, andwhich can be used for a single lens reflex camera.

The present invention will now be more apparent from the followingdescription referring to the embodiment shown in the drawing in which:

FIG. 1 is the cross sectional view of the lens system showing anembodiment of the present invention; and

FIG. 2 is the aberration curve thereof.

A wide angle lens of the back-telescopic type having a longer back focuswhen compared with the focal length may be considered by separating thelens into a lens group B, having the composite negative refractive powerand a lens group B having the composite positive refractive power at therear of the group B The group B has the effect to lengthen the backfocus and to increase the angle of field, but in view of aberration, itis apt to give the following drawbacks to the Whole lens system;

(1) Coma aberration is disturbed, and in particular it is easy to bringabout internal coma.

(2) Astigmatism is caused, and especially the sagittal image is easierto be curved in the direction of excessive correction, i.e., in thepositive direction than meridional image, and in the case of the angleof field over 60, this tendency becomes significant.

(3) It is easy to possess the negative distortion.

3,507,558 Patented Apr. 21, 1970 The positive lens group B corrects thewhole or a part of said tendency, and preferably corrects the finalimage. In accordance with the present invention, the following functionsare especially given to B (a) Correction of coma aberration.

(b) Correction of astigmatism.

When the corrections are to be carried out by the lens group B it isnecessary to enlarge the efiective diameter of the respective lensescomprising group B, which results in a complicated structure in which alarge number of lenses are required with the attendant disadvantages ofweight and size. v

According to the lens structure of the present invention, When viewedfrom the direction of the projection of light rays, the positive singlelens L comes first, and then the negative meniscus lens L, with theconvexed surface thereof facing the object is next. The lens group Bhaving negative refractive power is composed of these two lenses. Thepositive lens L is required for correction of distortion, and as to thepositive meniscus lens with the convexed surface thereof turned to theobject space, it is composed by cementing two lenses, i.e., biconvexlens L having a higher refractive power and a biconcave lens 1., havinga lower refractive power. When the refractive indices of L and L, areset up to be u, and m re spectively, they have the relation of n n andat the same time, the cemented surface r has a negative value and hasthe relation of eo |r 0.5f. These are aimed to correct the comaaberration of the light rays coming from outside of the main light raysprojected at a predetermined angle. When the value of r becomespositive, they do not work on the correction of coma aberration, andwhen the relation becomes 'r 0.5f, astigmatism is brought about, and themeridional image is curved in the negative which is difficult tocorrect.

Next comes the negative meniscus lens with the convex surface thereofturned to the space between the lens L and the stop. The negativemeniscus lens is obtained by cementing a biconcave lens L and biconvexlens L When the refractive indices of lenses L and L, are set to be m,and u and the radius of curvature of the cemented surface is r,;, thenthe conditions of n n and 3f r 0.25f are to be satisfied. Theseconditions not only prevent the light rays coming from inside of themain light rays and becoming coma flare but also correct the astigmatismby turning back the curve of sagittal image towards the positivedirection to the negative direction. When r, is larger than 3 it doesnot correct coma aberration. When'r is smaller than 0.25 the sagittalimage is excessively curved in the negative direction to deteriorate notonlyastigmatism but also coma aberration. and correction becomesdiificult. The composite focal length up to r is in the negative, andthis is remarkably eifective in increasing back focus.

The next two lenses, 'i.e., a positive meniscus lens L the convexsurface of which is toward the object and a biconvex lens L have thefunctions of making the virtual image formed by said lenses L through Linto a real image to form an excellent image on the surface of a film.The lenses L and L work to correct the spherical aberration. Thus, inaccordance with the present invention it is possible to produce a highlyefficient, small and light weight wide angle camera lens having a largeaperture ratio (such as Fz2), an angle of field over 60", and a longback focus, wherein aberration is excellently corrected throughout theimage surface. An example of thereof are What is claimed is:

1. A Wide angle camera lens having a large aperture ratio and long backfocus which consists of as viewed from the direction of the light raysincidence,

a positive convexo plano lens L M a negative meniscus single lens L withthe convex surface thereof turned to the object, d 1()833 a biconvexlens L and a biconcave lens L cemented r1=+3.2117 d 00978 n 53 9together in this order to form a doublet with the 1O convex surfacethereof directed to the object space T +1 3889 d =0. 06111 m=1. 0 andwith the cemented surface having positive power,

d1=0. 31111 7u=1. 51660 64.2 a PP dlaphragm, T4=+Q4778 d 0 22778 n 1 0 ablconcave lens L and a biconvex lens L cemented 5 5= together to form adoublet with the convex surface T= L5278 #129167 66'55 thereof directedto the image space and with the 0 9444 d =0. 14722 m=1. 51454 54.62cemented surface having negative power, a positive meniscus lens L; withthe convex surface 0 902 11. 0.02778 1.3 1.0 d d h ir t t (19:0.10444m=1. 78470 26.1 EC 6 to e lmage Space and r -1.0775 a blconvex lens LTw= 0 7172 2O 2. A wide angle camera lens having a large aperture 3 0556d =O. 08333 m1=10 ratio and a long back focus comprising in seriatim aposi- 5 6 tive lens, a negative meniscus single lens with its convex d.0. 00-78 m. 1.76684 4 .2 rum-140691 d 06944 MP1 0 surface toward the obect, a blconvex lens and a bicon- 15- m=+3. 6111 cave lens cemented toether w1th the convex surface d1- 71 1 74400 44 0 rected toward the obect, a sto a b1concave lens and a 7'14- 2.1958 P,

m5=10 blconvex lens cemented together w1th the convex surface thereofdirected toward the image, a positive meniscus I II IIIIV iv v Withregard to a wide anglecarnera lens having a large aperture ratio whichmay be used in a reflex camera, there are a number of problems such assize or form of the lens, coma aberrations and the like. In the presentinvention, attention was especially centered on the removal of comaaberration, so that sufiicient peripheral light can be utilized. It ispossible, in accordance with this invention, to produce a wide anglecamera lens of a large aperture ratio wherein various aberrations can beexcellently corrected so that it is possible to attain a back focus of1.13371 when the aperture ratio is over F/ 2 and the angle of field isover 60, and when the stop is opened, an excellent contrast can beobtained. Also it is possible to reduce the diameter of the lens in theforemost portion to a comparatively small size of about 1.3 thuspermitting the diameter of an attachment lens or filter to be made about1.5

convex lens, the lenses having the following relation: convex lens, thelenses having the following relati0-- d2=0. 00278 m=1. 71300 53.9 cc

ds=0. 22778 71 :1. 0 r +0. 8945 d8=0. 29167 na= 1. 60755 41. 9 r5= -1.5278 d =0. 14722 71 1. 51454 54. 62 T7 +0. 9444 65 da=0. 02778 m=1. 0

d =0. 19444 n 1. 78470 26. 1 r +1. 0775 dn=0. 08333 7L11= 1. 0 T11= -3.0550 a..=0. 00278 m,=1. 76684 46. 2

T12= 1. 0091 d13=0. 00944 7Z13=L0 ms= l. 0

5 6 where r r r are the radii of curvature of the FOREIGN PATENTSsurfaces, d d (1 are the axial thickness and air 962 368 7/1964 GreatBritain spacings of the lenses, n n n are the refractive indices, and isAbbes JOHN K. CORMIN, Primary Examiner References Cited 5 v CL UNITEDSTATES PATENTS 3502l5 1,955,590 4/1934 Lee 350-215

